Select the Theme drop-down list, select one. Click Preview to see if it's what you want. If not, repeat until you like one... if not, you may need to change your theme and choose Standard Theme depending how important this is.

Thanks for the tips about the pop up notification configurations. I did all what you suggested before I posted this. Nothing works. I was hoping to find an info where some text config files exist to tweak the settings.

I found Dunst replacement of the MATE notification daemon and it works much better (at least it is easy configurable by the config text file).